1. Field of the Invention
The present invention relates to a silicate-containing sheet. More particularly, the present invention pertains to a silicate-containing sheet which comprises a silicate having ultraviolet rays-attenuating effects, and photocatalytic actions such as antimicrobial actions and antifouling properties, and which is favorably employed in a field in which the aforesaid actions and properties are required.
2. Description of the Related Arts
It has heretofore been known that some kinds of metal oxides have ultraviolet rays scattering-actions, and they are employed as a raw material for cosmetics by taking advantage of such actions. For example, there are used finely pulverized titanium dioxide and zinc oxide, hybrid inorganic powders produced by compositely treating finely pulverized titanium dioxide with an iron oxide and finely pulverized hybrid inorganic powders produced by bringing selenium oxide and amorphous silica into composite form, as raw materials of cosmetics for finishing such as foundations, lipsticks and lip-creams.
On the other hand, it is known that titanium dioxide among a variety of metal oxides has both the above-mentioned ultraviolet rays scattering-action and a strong photocatalytic action. By the photocatalytic action for example optical energy is absorbed, thereby generating ionized oxygen molecules and as a result, an organic matter is subjected to oxidative decomposition. Examples of the hitherto known compounds having such a photocatalytic action(hereinafter sometimes referred to as "photocatalysL") include in the first place, titanium dioxide, strontium titanate (SrTiO.sub.3), barium titanate (BaTi.sub.4 O.sub.4), sodium titanate (Na.sub.2 Ti.sub.6 O.sub.13) cadmium sulfide, zirconium dioxide and .alpha.-Fe.sub.2 O.sub.3. Of these, titanium dioxide is typical.
In this connection, it is made possible to impart the surface of a material with a function of decomposing an organic matter by light irradiation only, provided that the aforesaid photocatalyst is in advance, applied to the surface of a material or incorporated in the coating on the surface of a material. The foregoing photocatalyst is begun to be put into practice in the fields of glass and tiles that are less apt to be stained, a toilet bowl and the like. In addition, research and development are carried out in a positive manner for a variety of functional products by utilizing the photocatalyst. For example, investigation is being made on the commercialization of, for example, (1) glass which is coated on the surface thereof, with a transparent photocatalyst, thus is capable of spontaneously decomposing the stain stuck to the surface thereof and also is imparted with an antimicrobial effect; (2) a building material capable of decomposing nitrogen oxides that are prime evils leading to air pollution by means of the solar optical energy; (3) an effluent water treatment agent comprising silica-gel particles coated with fine powders of titanium dioxide; and the like.
As mentioned hereinbefore, titanium dioxide has both an excellent ultraviolet rays scattering-action and a strong photocatalytic action. However when titanium dioxide is applied to or mixed into a plastic substrate in order to impart these functions to the plastic, a problem is caused in which the plastic substrate is deteriorated by the strong photocatalytic action of the titanium dioxide.
In order to solve such a problem, there has recently been developed a microcapsule in which titanium dioxide is covered with silica, and an attempt is made to impart deodorizing and antimicrobial functions by mixing the microcapsule in a plastic substrate. Since in the aforesaid microcapsule, the titanium dioxide is not exposed on the surface thereof, the deterioration of the plastic is suppressed, and an organic matter having small molecules can enter inside the microcapsule through numerous micropores that are present on the surface thereof, whereby the catalytic function is effectively exhibited. Nevertheless, the microcapsule of such a structure suffers from the disadvantage that troublesome operations are needed in the production thereof, thus bringing about a high production cost.
Moreover in the case of coating the surface of the substrate with an metal oxide such as titanium oxide, the resultant coating is readily peeled off therefrom if the metal oxide is applied as such. Such being the case, there is usually adopted a method in which a binder layer such as a silica film is placed on the surface of the substrate and then a mixture of the metal oxide and a binder is applied to the binder layer. However in the aforestated method, it is necessary to install a binder layer and also prepare a binder to be mixed with the metal oxide, thus bringing about a high production cost. Moreover, in the case where the metal oxide is titanium oxide and the substrate is a plastic some problems still remain unsolved, for example, it is inevitable that the deterioration of the substrate is accelerated even if a binder layer is placed on the surface of the substrate.
It being so, if it is possible to develop a specific substance which is imparted with ultraviolet rays-attenuating function, antimicrobial action and a photocatalytic function, which is applicable to a plastic substrate without causing the deterioration of the plastic substrate without needing a binder layer or a binder and further which is inexpensive, then it can be expected that the utilization of said substance in plastics drastically expands the usage of a functional product having ultraviolet rays attenuating properties, and photocatalytic functions such as antimicrobial properties and antifouling properties.